In six weeks Mercury falls 14,000,000 miles—­for it is a fall, though in a curve instead of a straight line—­falls 14,000,000 miles toward the sun!  And, as it falls, like any other falling body it gains in speed, until, having reached the perihelion point, its terrific velocity counteracts its approach and it begins to recede.  At the end of the next six weeks it once more attains its greatest distance, and turns again to plunge sunward.

Of course it may be said of every planet having an elliptical orbit that between aphelion and perihelion it is falling toward the sun, but no other planet than Mercury travels in an orbit sufficiently eccentric, and approaches sufficiently near to the sun, to give to the mind so vivid an impression of an actual, stupendous fall!

Next let us consider the effects of this rapid fall, or approach, toward the sun, which is so foreign to our terrestrial experience, and so appalling to the imagination.

First, we must remember that the nearer a planet is to the sun the greater is the amount of heat and light that it receives, the variation being proportional to the inverse square of the distance.  The earth’s distance from the sun being 93,000,000 miles, while Mercury’s is only 36,000,000, it follows, to begin with, that Mercury gets, on the average, more than six and a half times as much heat from the sun as the earth does.  That alone is enough to make it seem impossible that Mercury can be the home of living forms resembling those of the earth, for imagine the heat of the sun in the middle of a summer’s day increased six or seven fold!  If there were no mitigating influences, the face of the earth would shrivel as in the blast of a furnace, the very stones would become incandescent, and the oceans would turn into steam.

Still, notwithstanding the tremendous heat poured upon Mercury as compared with that which our planet receives, we can possibly, and for the sake of a clearer understanding of the effects of the varying distance, which is the object of our present inquiry, find a loophole to admit the chance that yet there may be living beings there.  We might, for instance, suppose that, owing to the rarity of its atmosphere, the excessive heat was quickly radiated away, or that there was something in the constitution of the atmosphere that greatly modified the effective temperature of the sun’s rays.  But, having satisfied our imagination on this point, and placed our supposititious inhabitants in the hot world of Mercury, how are we going to meet the conditions imposed by the rapid changes of distance—­the swift fall of the planet toward the sun, followed by the equally swift rush away from it?  For change of distance implies change of heat and temperature.